Abstract
Introduction
Corrosion of the femoral head-trunnion junction in modular hip components has become a concern as the corrosion products may lead to adverse local tissue reactions. A simple way to avoid trunnion corrosion is to manufacture the femoral head with a non-metallic material, such as ceramics that are widely. An alternative solution may lie in advanced polymers like polyaryletherketones (PAEKs). These thermoplastics have high mechanical strength necessary for use as femoral heads in hip arthroplasty, but they must be tested to ensure that they do not adversely affect the wear of the ultrahigh molecular weight polyethylene (UHMWPE) liner counterface. Pin-on-disc (POD) wear testing has been extensively used to evaluate the wear properties of UHMWPE prior to more extensive and costly analysis with joint simulators. We hypothesized that the wear of crosslinked UHMWPE would not be adversely affected in POD tests when articulated against an advanced thermoplastic counterface.
Methods
0.1 wt.% VitE blended UHMWPE stock was e-beam irradiated to 100, 125, 140, 160, and 175 kGy and machined into cylindrical pins for testing. An additional group of 100 kGy e-beam irradiated and melted UHMWPE (with no vitamin E) was also machined and tested.
Three different counterface materials were tested: (1) Cobalt-chrome (CoCr) with a surface roughness (Ra) of <0.5 μm, (2) Biolox™ ceramic (CeramTec), and (3) Polyetheretherketone (PEEK), a member of the PAEK family (Fig 1).
A bidirectional POD wear tester [1] was used to measure the wear rate of UHMWPE specimens, where the specimens moved in a 10 mm × 5 mm rectangular pattern under a Paul-type load curve [2] synchronized with the motion. The peak load of the loading curve corresponded to a peak contact pressure of 5.1 MPa between each UHMWPE pin specimen and the counterface disc. Each test was conducted at 2 Hz in undiluted bovine serum stabilized with ethylenediamine tetraacetate (EDTA) and penicillin. The pins were cleaned and weighed daily, and a wear rate was calculated at the end of each test by linear regression.
Results
As expected, higher radiation doses led to lower wear rates against all counterface materials (Fig 2). The PEEK discs produced the lowest UHMWPE wear in each group and the CoCr discs produced the highest UHMWPE wear; however, the two UHMWPE groups with the lowest wear rates showed no difference between the three counterface materials.
Conclusions
Even though the PEEK discs had visible machining marks – that is they were not polished to an implant surface finish – they still yielded the lowest wear rates for UHMWPE articulating against them when compared to the highly polished and smooth CoCr and ceramic materials. Implementing further steps to better the surface roughness of the PEEK counterface may yield even better wear rates. Using PEEK in femoral heads may alleviate issues with trunnion corrosion without increasing the incidence of osteolysis or other wear related issues.
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